When descending the leader should never descend past tangles, and should always deal with problems when they are in front of them. If they don’t they risk either pulling down rocks while pulling the loops down, or getting stuck with the rope tangled above them and out of reach.
Returning back up the rope
If you’re forced to jug back in order to find something better then use your prussic loops rapped around both ropes, taking in and tying off your descender as you gain height. If you’re using mechanical ascenders and suspect you may have to come back up again, then after threading the ropes through the anchor as normal, clip them in with a figure of eight on each strand. This means that if the leader needs to jumar back up the rope then they can do so safely on a single strand of rope using mechanical ascenders – which are much quicker then prussic loops. The figure of eight can be untied once the leader is ready at the next stance to carry out a test pull.